The experiments in this proposal are designed to investigate the effects of neurotoxins, poisons, local anaesthetics, quaternary amines, and other blocking agents on the intracellular responses of photoreceptors. These experiments are designed to permit a pharmacological dissection of receptor responses, in order to reveal the number of different kinds of ion channels in receptor membranes and the way these channels interact. The effects of divalent cations will also be studied. Finally, an attempt will be made to measure the ion specificities of receptor channels. The purpose of these experiments is two-fold: First, to discover how changes in membrane permeability produce receptor responses, and second, to begin an investigation of the pharmacology of photoreceptors which will, we hope, assist in the evaluation of drugs for the treatment of retinal diseases.